[Tech Talk] Foreign Grain Beetles and Plaster Beetles: Stored-Product Pest “Imposters”

As pest management providers, we’ve all seen this scenario more than once:

A food-processing client tells you that a load of their product — flour, rice, dried milk, animal feed, you name it — arrived at its end destination, and was rejected due to stored product pests that were seen on the boxes, bags or totes of product. Maybe the destination was across town, maybe it was somewhere in North America, or maybe it traveled halfway around the world, only to be shipped back to the producer at great expense.

The client’s customer sent back pictures of small brown beetles as justification for rejecting the load, and the product is now on its way back to the production plant, via semi-trailer, rail or piggyback ocean container. The pictures may or may not be clearly defined, and may or may not have a size reference, such as a ruler, alongside the pictured insects. In any case, someone is going to pay.

Sadly, in some situations like this, the headaches and expenses of rejecting a load of product and sending it back the way it came are unnecessary. A little bit of client education can go a long way.

Foreign grain beetles (Ahasverus advena) and plaster beetles (Family Lathridiidae) are often the culprit when insects erroneously assumed to be stored product pests show up on shipments of food products.

Biological Background. Foreign grain beetles are scavengers that have a taste for mold, among other food sources. When found in grain, the grain is usually damp or moldy. They are medium-brown beetles about 1/12 inch long — much smaller than the confused or red flour beetles that they somewhat resemble, and for which they might be mistaken. They can be positively identified by the protruding lobes at the front corners of their pronotum (the shield-like structure covering the thorax), and by their three-segmented antennal club. They are sometimes pests of grain, but more often become pests when they enter buildings in late summer, attracted by lights and the prospect of winter harborage.

Plaster beetles also are called minute brown scavenger beetles (“minute” in reference to the beetle’s diminutive size). They are exclusively fungus feeders. Like foreign grain beetles, they are very small, are attracted to lights, and are excellent fliers. They can be distinguished by their small size (as small as 1/32 inch in length), and by the shape and size of their pronotum (always narrower than the abdomen). Most plaster beetles have rows of punctures on their wing covers.

Both foreign grain beetles and plaster beetles live happily outdoors all summer long. Then, as the nights begin to get cool, they head for warmer places, using their strong attraction to light and their excellent flying skill to lead them to structures. This occurs earlier in northern regions, and later in the south.

Food Plant Havoc. The real trouble begins when employees of food-processing plants, especially those running three shifts, become careless with insect exclusion. Often, these facilities are uncomfortably warm during late-summer and early-autumn evenings, and night-shift employees open dock doors to let in some cool air. Adjacent to all of those open dock doors is the packaging department, where food, feed and ingredients are placed on pallets. The packages — bags, boxes or totes — are stabilized using a rotating platform that encases the pallet and its contents in clear, strong plastic.

Foreign grain beetles and plaster beetles, hard-wired by nature to seek warmth and light — and blessed with the gift of flight — make a beeline straight for the light, eventually finding their way to the packaging department and the even brighter lamps illuminating the pallet-wrapping operations. Many of them fall or land on the boxes, bags or nylon-fabric totes just as the pallet wrap is being applied, and become trapped there. Along comes a forklift and loads them onto a trailer, boxcar or shipping container.

And that’s how things are when the customer opens the doors of the shipping vessel: hundreds of small insects cover the packages of product, and the customer jumps to conclusions. In the best case scenario, the customer might have the product, which they believe to be infested, fumigated and pass this cost on to the producer. But less knowledgeable customers may put the container back on the ship and send it off to sea — an expensive round trip.

Prevention. In descending order of importance, here are four things you can advise your food processing clients to do in late summer and fall to avoid dealing with these returned shipment situations:

Keep doors and windows closed or screened. Industrial-grade screens are available that fit into the track hardware of dock doors, and snug up against the door frame to keep insects out. Depending on the mesh size chosen, they may not work perfectly (foreign grain beetles and plaster beetles are very small and can make their way through even a 12-mesh screen); but they will help a great deal. Screens made exclusively for food plant dock doors are available and may be a little pricey; but the cost of return freight from Singapore or São Paolo if a load is rejected is much worse.

Install appropriate lighting. Over dock doors, frequently used entrance and exit doors and in plant locations adjacent to doors to the exterior, sodium-vapor lamps create a wavelength of light that is much less attractive to night-flying insects than incandescent, fluorescent or mercury-vapor lamps. This can help keep the pests away.

Check the facility’s air-intake and exhaust system. All food plants are very careful about the air that they bring into the building. But periodic inspection of air-handling units on the roof or side of the building might reveal gaps that potentially allow for pests to get inside — for example, filter panels that are loose or are not properly fitted into their frames.

And in many buildings, exhaust air is expelled through virtually wide-open pipes. These are usually equipped with screens that will keep a bird from flying inside the the building, but not a flying insect. These, too, must be properly screened, or facility management must ensure that all production areas are under continuous positive pressure. Turn off the exhaust system for one minute, and the building begins sucking insects inside.

Screening of intake and exhaust air is a complex engineering challenge, because screens affect a building’s ability to inhale or exhale efficiently. Educate your client about the need for effective exclusion, and leave it to the plant’s engineers to work out a way of keeping insects out while still being able to handle the air they need.

Inspect for conducive conditions indoors. Especially in the case of plaster beetles, the real source of these pests is sometimes indoors. If warehouse floors have expansion gaps between poured sections and along walls, dust and other debris can sift into the cracks, creating a moist, fungus-laden growing medium for mold-loving beetles like plaster beetles. If you find dirty floor cracks and expansion joints packed with damp, putty-like debris, ask your client to get these cleaned out, and then consider applying an appropriately labeled insecticide to the cracks.

If your client really wants to get serious about expansion joints, flexible sealants are available that will do the trick. Just don’t tell your client, “Hey, you’ve got to caulk those expansion joints,” leading the client to believe it’s an easy fix they can accomplish themselves. They might send someone down to the local big-box home-improvement store for a case of silicone caulk, when what they really need to do is consult with an engineering firm or other qualified professional to recommend a durable, flexible and cleanable sealant.

Summary. As small as they are, foreign grain beetles and plaster beetles can create a lot of sorrow and lost revenue. Armed with some knowledge and good advice for your clients, you can help them avoid losses due to fungus feeders that might be mistaken for stored product pests.

The author is technical director at Plunkett’s Pest Control, Fridley, Minn., and a member of the Copesan Technical Committee. He can be reached at jbruesch@giemedia.com.

Copesan is an alliance of pest management firms serving national accounts throughout North America. To learn more, visit www.copesan.com.